The present invention relates to an evaporative heat rejection method and apparatus, such as a condensing apparatus for use with air conditioners and other applications where cooling below ambient temperatures is required. Evaporative condensing substantially improves the efficiency of the air conditioner.
Air conditioners and other devices that need to reject heat at ambient conditions typically use air or water for this purpose. This may be done in one of the following ways:                1. Air at ambient temperature is passed over the heat exchanger containing the medium to be cooled. The Carnot cycle for such a system is depicted by the plot 1c, 2c, 3c, 4c in FIG. 1. For the refrigerant R-22, condensation takes place at 125° F.        2. Water from a city supply or other source is passed in a once-through system through a heat exchanger containing the medium to be cooled. The water leaving the heat exchanger is sent to the city drain or a lake.        3. Water is passed through heat exchanger containing medium to be cooled. After it cools the medium, this water is sent to a device like a cooling tower where it is cooled by evaporating a portion of water in the ambient air stream. Cooled water is then sent back through the heat exchanger in a continuous loop. Make up water is added to the system to compensate for three modes of losses:                    a. Water that is evaporated to cool the rest of the water.            b. Water that is lost to the air stream.            c. Water that is bled off to maintain the dissolved solids to an acceptable level. This amount of bleed off is determined by the level of dissolved solids in the available replacement water and maximum level of dissolved solids in the water to keep the maintenance to acceptable level. The Carnot cycle for this system is depicted by the plot 1b, 2b, 3b, 4b in FIG. 1. For the refrigerant R-22, condensation takes place at 105° F.                        4. Water is evaporated on the surface of the heat exchanger containing the medium to be cooled. A large amount of water is sprayed on the heat exchanger surface, and ambient air is also forced over the heat exchanger containing fluid to be cooled. A very small portion of the water (about 1%) evaporates in the air, taking heat from the fluid inside the heat exchanger. Excess water is collected in a container, typically a water basin at the foot of the heat exchanger, where make up water is added to the system. This excess water and make up water is recirculated through the heat exchanger. The make up water is added to compensate for the same loss modes mentioned above.        5. Water is evaporated on the surface of the heat exchanger containing the medium to be cooled, and ambient air is also forced over the heat exchanger. The heat exchanger is covered with a water absorptive material. The amount of water sprayed on the heat exchanger is completely evaporated in cooling the medium to be cooled. No liquid water leaves the heat exchanger.        6. This method is in accordance with the present invention, and is not to be considered prior art. Water is evaporated on the surface of the heat exchanger containing the medium to be cooled, and ambient air is forced over the heat exchanger. Water evaporates in the ambient air, cooling the medium inside the heat exchanger. The heat exchanger is covered with a water absorptive material. The amount of water deposited or sprayed on the evaporator is equal to the sum of the water that is evaporated to cool the medium inside heat exchanger, and the water that is lost to the air stream.        
Excess water that leaves a heat exchanger without being evaporated carries concentrated dissolved substances away from the heat exchanger surface.
As noted, air at 35° C. (95° F.) dry bulb temperature and 23.89° C. (75° F.) wet bulb temperature may be expected to have a refrigerant condensing temperature of 51.67° C. (125° F.), in air only, using the method described in paragraph 1 above. Whereas water cooled and evaporative cooled equipment described in paragraphs 2 through 5, using the same air may be expected to have a refrigerant condensing temperature of 40.56° C. (105° F.). The Carnot cycle of the paragraph 6 method is depicted as 1a, 2a, 3a, 4a in FIG. 1, for a condensing temperature of 100° F., noticeably lower than for the other methods.
The method described in paragraph 1 also requires larger amount of air drawn over the heat transfer surface to carry the heat away, compared with that required in applications described in paragraphs 2 through 6. However, using water to cool in a once through system as described in paragraph 2 requires a large amount of water to be wasted. The cooling tower and recirculation described in paragraph 3 requires additional pumping capacity, additional fan capacity, and considerable maintenance (in terms of chemical additives and physical cleaning) to avoid formation of algae or other microorganisms and to avoid formation of scales. Scale reduces the heat transfer efficiency, or cause corrosion of the heat exchanger. Evaporative condensing as described in paragraph 4 eliminates the need for another device like a cooling tower while achieving temperatures in the medium to be cooled comparable to those obtained by using recirculated water. However, conventional evaporative condenser designs use recirculated water, thus causing maintenance problems previously described with respect to cooling towers. Certain inventions in recent past, described in paragraph 5, have proposed the use of a once-through system of water where water is completely evaporated in the evaporative condenser. This method leaves deposits of minerals previously dissolved in water on the heat transfer surface used, thus reducing the heat transfer efficiency of the heat exchanger and requiring elaborate cleaning and/or replacement of the heat transfer surface.